• Journal of the Korea institute for structural maintenance and inspection
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• v.19 no.6
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• pp.63-71
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• 2015

The objective of the present study is to investigate how elevated temperature ranging from $100^{\circ}C$ to $800^{\circ}C$ as well as room temperature affects the variation of mechanical properties of high strength concrete ($over\;f_{ck}=60MPa\;grade$). In this experiment, specimens were exposed for a period of $2^{\circ}C/min$ to temperatures of $20^{\circ}C$, $100^{\circ}C$, $200^{\circ}C$, $300^{\circ}C$ $400^{\circ}C$, $500^{\circ}C$, $600^{\circ}C$, $700^{\circ}C$ and $800^{\circ}C$, respectively. Accordingly, the study investigated the fire resistance performance of high strength concrete mixed with composite fibers which composed with hybrid fibers and steel fibers. After cooling down to ambient temperature, the following basic mechanical properties were then evaluated and compared with reference values obtained prior to thermal exposure: (i) compressive strength in room temperature; (ii) residual compressive strength; (iii) Poisson's ratio; (iv) weight change; (v) SEM analysis & XRD analysis In addition, XRD and SEM Images analyses were performed to investigate chemical and physical characteristics of high strength concrete with composite fibers according to high temperature.

• Composites Research
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• v.28 no.6
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• pp.372-377
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• 2015

In this study, we analyzed process numerically when a bogie frame skin is manufactured by applying resin transfer molding process using composite material instead of steel. Processing time was compared based on the various thickness of bogie frame skin and the weight variation of a skin was also considered. As a result, RTM processing time decreases and the weight of a bogie reduces as the thickness of frame skin increases with the assumption that fiber volume is constant inside the skin. By considering these results as the information to estimate the production cost, trade-off between two fields, processing time and structural properties, can be performed in design optimization to produce bogie frame.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.1045-1048
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• 2008

Fiber reinforced polymer (FRP) usually exhibits inherent brittleness under tensile stress. Application of FRP tendons to concrete beam leads to undesirable flexural behavior due to limited ductility compared to prestressed concrete beam with steel tendons. It has been experimentally observed that partial improvement of flexural behavior can be achieved by releasing FRP tendons' strain by unbonding FRP tendons. In order to estimate and apply the degree of improvement to the design, reasonable yet practical model predicting flexural strength as well as overall flexural behavior of unbonded FRP prestressed concrete beam is needed. In this study, an elaborated model in describing curvature distributions and flexural strength at ultimate stage of unbonded FRP tendons is described. There have been close agreements on the flexural strength of the FRP prestressed concrete beam between the predictions by nonlinear computer program and by the model.

• International Journal of Concrete Structures and Materials
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• v.11 no.1
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• pp.135-149
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• 2017

The prediction of the actual ultimate capacity of confined concrete columns requires partial confinement utilization under eccentric loading. This is attributed to the reduction in compression zone compared to columns under pure axial compression. Modern codes and standards are introducing the need to perform extreme event analysis under static loads. There has been a number of studies that focused on the analysis and testing of concentric columns. On the other hand, the augmentation of compressive strength due to partial confinement has not been treated before. The higher eccentricity causes smaller confined concrete region in compression yielding smaller increase in strength of concrete. Accordingly, the ultimate eccentric confined strength is gradually reduced from the fully confined value $f_{cc}$ (at zero eccentricity) to the unconfined value $f^{\prime}_c$ (at infinite eccentricity) as a function of the ratio of compression area to total area of each eccentricity. This approach is used to implement an adaptive Mander model for analyzing eccentrically loaded columns. Generalization of the 3D moment of area approach is implemented based on proportional loading, fiber model and the secant stiffness approach, in an incremental-iterative numerical procedure to achieve the equilibrium path of $P-{\varepsilon}$ and $M-{\varphi}$ response up to failure. This numerical analysis is adapted to assess the confining effect in rectangular columns confined with conventional lateral steel. This analysis is validated against experimental data found in the literature showing good correlation to the partial confinement model while rendering the full confinement treatment unsafe.

• International Journal of Concrete Structures and Materials
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• v.11 no.2
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• pp.327-341
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• 2017

Openings in slabs are usually required for many different applications such as aeriation ducts and air conditioning. Opening in concrete slabs due to cutouts significantly decrease the member stiffness. There are different techniques to strengthen slabs with opening cutouts. This study presents experimental and numerical investigations on the use of Carbon Fiber Reinforced Polymers (CFRP) as strengthening material to strengthen and restore the load carrying capacity of R.C. slabs after having cutout in the hogging moment region. The experimental program consisted of testing five (oneway spanning R.C. flat slabs) with overhang. All slabs were prismatic, rectangular in cross-section and nominally 2000 mm long, 1000 mm width, and 100 mm thickness with a clear span (distance between supports) of 1200 mm and the overhang length is 700 mm. All slabs were loaded up to 30 kN (45% of ultimate load for reference slab, before yielding of the longitudinal reinforcement), then the load was kept constant during cutting concrete and steel bars (producing cut out). After that operation, slabs were loaded till failure. An analytical study using finite element analysis (FEA) is performed using the commercial software ANSYS. The FEA has been validated and calibrated using the experimental results. The FE model was found to be in a good agreement with the experimental results. The investigated key parameters were slab aspect ratio for the opening ratios of [1:1, 2:1], CFRP layers and the laminates widths, positions for cutouts and the CFRP configurations around cutouts.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.29-32
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• 2008

The renewal of existing buildings rather than new construction has increased due to it's cost effective characteristics. Remodeling is also an environmentally-friendly approach that reduces the amount of waste in construction site. Remodeling can sometimes include partial destruction of the structural members of a building. In addition it is important that the buildings under going remodeling retrofitted to make themselves stable and meet up with the future demands for better structural performance. The objective of this paper is to present the test results and structural behavior of RC walls that are perforated and to introduce effective retrofitting methods by evaluating efficacy of passive retrofit and active retrofit. Passive retrofit and Active retrofit using carbon fiber sheets, steel plates and wire that are widely used for strengthening the main members of existing buildings. The test results showed that the failed specimens had shear fractures and that two different types of retrofit method had different effects on the strengths of each specimen.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.441-444
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• 2008

Ultra high performance concrete (UHPC) in this study is composed of sand, cement, silica fume, siliceous powder, superplasticizer and steel fiber. UHPC is composed of fine mineral particles below 0.5mm in diameter. In general, siliceous powder improves the mechanical properties of concrete by physical and chemical effect. Physical effect is related with filling interior voids which weaken the mechanical properties and chemical effect with reaction of $SiO_2$ with cement hydrates in a condition of high temperature and pressure. We evaluated the effect of siliceous powder's particle size on the mechanical properties of ultra high performance concrete in air pressure and $90^{\circ}C$ steam curing condition. siliceous powder's particle size in this study is in the range of $2{\mu}m$ to $26{\mu}m$. Fluidity in a fresh concrete, compressive strength, ultimate strain, elastic modulus and flexural strength in a hardened concrete was evaluated. We could find out that the smaller siliceous powder's particle size is, the better the fluidity and strength properties.

• Smart Structures and Systems
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• v.17 no.1
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• pp.91-105
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• 2016

One of the most commonly used techniques to strengthen steel reinforced concrete structures is the application of externally bonded patches in the form of carbon fiber reinforced polymers (CFRP) or recently, textile reinforced cements (TRC). These external patches undertake the tensile stress of bending constraining concrete cracking. Development of full-field inspection methodologies for fracture monitoring are important since the reinforcing layers are not transparent, hindering visual observation of the material condition underneath. In the present study acoustic emission (AE) and digital image correlation (DIC) are applied during four-point bending tests of large beams to follow the damage accumulation. AE helps to determine the onset of fracture as well as the different damage mechanisms through the registered shifts in AE rate, location of active sources and change in waveform parameters. The effect of wave propagation distance, which in large components and in-situ can well mask the original information as emitted by the fracture incidents is also discussed. Simultaneously, crucial information is supplied by DIC concerning the moments of stress release of the patches due to debonding, benchmarking the trends monitored by AE. From the point of view of mechanics, conclusions on the reinforcing contribution of the different repair methodologies are also drawn.

• Steel and Composite Structures
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• v.18 no.3
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• pp.583-601
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• 2015

In the present study, the combined effects of thermal and mechanical loadings on the interlaminar shear stresses of both moderately thin and thick composite laminated beams are numerically analyzed. The finite element modelling of laminated composite beams and analysis of interlaminar stresses are performed using the commercially available software package MSC NASTRAN/PATRAN. The validity of the finite element analysis (FEA) is demonstrated by comparing the experimental test results obtained due to mechanical loadings under the influence of thermal environment with those derived using the present FEA. Various parametric studies are also performed to investigate the effect of thermal loading on interlaminar stresses generated in symmetric, anti-symmetric, asymmetric, unidirectional, cross-ply, and balanced composite laminated beams of different stacking sequences with identical mechanical loadings and various boundary conditions. It is shown that the elevated thermal environment lead to higher interlaminar shear stresses varying with the stacking sequence, length to thickness ratio, ply orientations under identical mechanical loading and boundary conditions of the composite laminated beams. It is realized that the magnitude of the interlaminar stresses along xz plane is always much higher than those of along yz plane irrespective of the ply-orientation, length to thickness ratios and boundary conditions of the composite laminated beams. It is also observed that the effect of thermal environment on the interlaminar shear stresses in carbon-epoxy fiber reinforced composite laminated beams are increasing in the order of symmetric cross-ply laminate, unidirectional laminate, asymmetric cross-ply laminate and anti-symmetric laminate. The interlaminar shear stresses are higher in thinner composite laminated beams compared to that in thicker composite laminated beams under all environmental temperatures irrespective of the laminate stacking sequence, ply-orientation and boundary conditions.

• Steel and Composite Structures
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• v.25 no.1
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• pp.105-116
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• 2017

This paper presents the dynamic stability study of laminated composite plates with different force combinations and aspect ratios. Optimum non-diverging stacking is obtained for certain loading combination and aspect ratio. In addition, the stability force is maximized for a definite operating frequency. A dynamic version of the principle of virtual work for laminated composites is used to obtain force-frequency relation. Since dynamic stiffness governs the divergence or flutter, an efficient optimization method is necessary for the response functional and the relevant constraints. In this way, a model based on the ant colony optimization (ACO) algorithm is proposed to search for the proper stacking. The ACO algorithm is used since it treats with large number of dynamic stability parameters. Governing equations are formulated using classic laminate theory (CLT) and von-Karman plate technique. Load-frequency relations are explicitly obtained for fundamental and secondary flutter modes of simply supported composite plate with arbitrary aspect ratio, stacking and boundary load, which are used in optimization process. Obtained results are compared with the finite element method results for validity and accuracy convince. Results revealed that the optimum stacking with stable dynamic response and maximum critical load is in angle-ply mode with almost near-unidirectional fiber orientations for fundamental flutter mode. In addition, short plates behave better than long plates in combined axial-shear load case regarding stable oscillation. The interaction of uniaxial and shear forces intensifies the instability in long plates than short ones which needs low-angle layup orientations to provide required dynamic stiffness. However, a combination of angle-ply and cross-ply stacking with a near-square aspect ratio is appropriate for the composite plate regarding secondary flutter mode.